In designing an image capture and reproduction system, it is important to be able to determine the magnitude of the level of image degradation to be expected in the final image as viewed by the observer. Understanding the magnitude of the image degradations due to grain is also important to the use of the image reproduction system and can have a major impact on the selection of key elements for use in the imaging chain. For example, in a photographic system, the selection of a film speed, film format, and film type are determined by the image to be captured and the end use of the final image. The film grain also becomes important depending on the degree of enlargement anticipated for the final print.
In a photographic system, the variations in otherwise uniform responses to exposing light are referred to as grain. These variations in the density can be observed through physical measurement by measuring the optical density of photographic materials, such as film or paper, with a microdensitometer. The root mean square (rms) value or standard deviation is used as a measure of the variation in density of an otherwise uniform area. This value is referred to as the granularity. A photographic image is perceived by an observer and the perception of these unwanted, random fluctuations in optical density are called graininess. Thus, the physically measured quantity of granularity is perceived by the observer as a level of graininess.
The first grain slide or ruler was designed and fabricated by Thomas Maier et al. (See for example, T. O Maier and D. R. Miller, "The Relationship Between Graininess and Granularity" SPSE's 43 Annual Conference Proceedings, SPSE, Springfield, Va. pp207-208, (1990)). The fundamental relationship relating the granularity and graininess was determined by C. James Bartleson (See for example, C. J. Bartleson, The Journal of Photographic Science, 33, pp117-126, (1985)). He determined the following relationship between the graininess G.sub.i and the granularity .sigma..sub.v EQU G.sub.i =a*log(.sigma..sub.v)+b Eq. (1)
where a and b are constants. He also determined that the perceived graininess did not depend on the color of the image, thus graininess was found to be strictly a function of the achromatic channel of the visual system.
Maier et al. produced a series of uniform neutral patches of grain at the same average density with increasing amounts of grain using a digital simulation instrument. They then used microdensitometer measurements and the fundamental psychophysical relationship to relate the graininess to the rms granularity. This was accomplished by assuming that a 6% change in granularity would correspond to a 2 unit change in graininess, or grain index. As a result of this assumption, the constant multiplying the lead term must be 80 since the log range of the ruler patches was 1.2 or about 48 times log of (1.06). They then assumed the lowest patch was grainless and assigned it an arbitrary value of 25. The following equation resulted EQU G.sub.i =80*log(.sigma..sub.v)-28.64 Eq. (2)
Then a series of 18 uniform neutral samples of increasing grain were assigned train index numbers in 17 unequal steps from 25 to 120 depending on the measured granularity. The final grain ruler consisted of two scales printed on black and white photographic paper mounted on a rigid backing material.
The resulting grain ruler was then used as a scaling tool to evaluate the graininess in other photographic materials. Such other materials consisted primarily of photographic materials with either uniform areas or images printed on them. In the form described above the grain ruler suffers from several significant deficiencies.
In use on contemporary photographic materials, the ruler led to widely divergent measurements by individual users. Measurements on colored photographic materials led to the most widely varying results. Since most current photographic materials are colored in nature, this is a serious deficiency. The non uniform scale of the original ruler, the arbitrary range of sample grain levels, and the layout as two separate rulers led to further difficulties in use. In addition, the method of generating the ruler failed to take into account the different look that grain has in different imaging systems, and did not address how one might model or display the impact grain would have in images rendered in media and materials other than silver halide photographic materials. The display of grain rendered in video and other modern optoelectronic output devices was also not addressed.